The number of ways to deliver telephone service continues to grow. “Plain old telephone service” (POTS), comprising analog telephones connected via pairs of copper wires to the public switched telephone network (PSTN), is now complemented by, for example, cellular wireless telephony, voice over internet protocol (VoIP), WiFi VoIP clients, and satellite-based telephony, among other services. A consequence is that people can remain in contact in places and circumstances where and when it was previously impossible.
Because people can be reachable, however, more and more people are expected to do so. Each form of telephone service can require a separate telephone number, line, or both, and the result can be that a caller must attempt calls to several different numbers before reaching the desired person. It is conceivable, for example, that a single person may have one phone number at work, one or more phone numbers at home, a personal mobile phone number, a work mobile phone number, and a phone number corresponding to a VoIP softphone.
FIG. 1 is a block diagram illustrating interconnected components of a telephone network 100, such as may be found in the prior art. The network 100 includes the public switched telephone network (“PSTN”) 105, which is the worldwide set of interconnected telephone networks that deliver fixed telephone services to the general public and are usually accessed by telephones and private branch exchanges (“PBX”), transmitting voice, other audio, video, and data signals.
The PSTN 105 comprises a plurality of interconnected switches. The PSTN 105 is formed from the interconnection of e.g., third-party switch 106, provider switch 107, and other switches (not pictured) within the PSTN 105. Fast digital links, referred to as “trunks” 108, connect switches in most of the PSTN 105, although some older analog equipment remains in use.
To connect something, e.g., the provider's switch 107, to the PSTN 105, typically means to connect it via a trunk 108 to another switch (not pictured) within the PSTN. Consistent with the usage common in the art, however, systems will be described herein as connected to the PSTN 105 as such, with the existence of a plurality of interconnected switches (not pictured) within the PSTN 105 left implicit. FIG. 1 accordingly shows the third-party switch 106 and provider's switch 107 separate from the PSTN 105, although they are part of it.
The PSTN 105 comprises analog phones, which participate in the PSTN 105 via switches. As depicted in FIG. 1, an analog phone 110 connects via a pair of copper wires 111 to the third-party switch 106. Similarly, other analog phones 112 connect via pairs of copper wires 113 to the provider's switch 107. Because they exist on the edges of the PSTN 105, analog phones may be referred to as “edge devices.”
An analog phone 112 is so called because it sends and receives analog electrical signals that represent voice data. Internally, however, the PSTN 105 primarily uses digital switches and digital trunks. Thus, a provider's switch 107 converts between the analog data used by the analog phone 112a and the digital data used internally by the switch 107 and exchanged within the PSTN 105.
Although not depicted in FIG. 1, switches within the PSTN 105 exist in a hierarchy. At the lowest level of the hierarchy are switches, such as the third-party switch 106 and the provider's switch 107, that connect both to the PSTN 105 and to analog telephones. For historical reasons, these switches, which connect both to edge devices and to other switches, are often called “class 5 switches.” A single class 5 switch may serve hundreds or thousands of telephone lines.
Higher-level switches (not pictured) may connect only to other switches and may be used, e.g., to establish a connection between different geographic areas. To handle telephone calls between different metropolitan areas, an inter-exchange carrier (“IXC”) connects to the PSTN 105 directly or indirectly. An IXC maintains its own facilities, such as, e.g., trunks and switches (not pictured), to route telephone calls. A provider 115 of telecommunications services may have its own IXC facilities 121 and may connect to the PSTN 105 directly or, as depicted in FIG. 1, through an interface and control system 125. A third-party carrier may also provide an IXC 120, which may connect directly to the PSTN 105.
To enable forms of telephone service other than POTS to interoperate with the PSTN 105, a provider 115 of telecommunications services may connect, e.g., one or more switches, gateways, or both to the PSTN 105. As depicted in FIG. 1, a provider 115 has connected a switch 107 to the PSTN 105. Besides the connections 113 to analog phones 112, the provider's switch 107 has a connection via a dedicated digital trunk 116 to a private branch exchange (“PBX”) 121. A PBX 121 may provide switching, signaling, and/or analog-to-digital conversion functions for multiple analog phones 122, digital phones 123, or both and typically exists within a large organization with many telephone lines. A PBX 121 may allow calling within the organization to take place without use of any facilities of the PSTN 105.
A provider 115 may use an interface (or gateway) system 125 to bridge telephony over the PSTN 105 and VoIP transmitted across a packet-switched data network 126, such as the Internet. The interface system 125 connects to the IP network 126, e.g., via an Ethernet cable 127 connected to a router (not pictured). A VoIP client, such as, e.g., a residential analog telephone adapter 128 or a commercial VoIP PBX 129 also connects to the IP network 126, exchanging data packets with the interface system 125. Other VoIP clients may use the network 126, such as a VoIP client 130 that uses WiFi to connect wirelessly to the network 126, or a computer program (called a “soft client”) that executes on a networked computer 131.
The interface system 125 also connects to the PSTN 105 via a trunk 108c. The interface system 125 supports calls among VoIP clients by, e.g., providing directory and/or addressing information. The interface system 125 also enables calls to cross between the PSTN 105 and the IP network 126 by, e.g., translating data and signaling protocols.
Other services may also be provided through the PSTN 105. For example, a provider of wireless telephone services 135 may also connect to the PSTN 105 through a digital trunk 108e. 
A telephone call may begin from, e.g., an analog phone 112a connected to the provider's switch 107. The switch 107 detects the dialed number and establishes the call. If the destination of the call is another analog phone 112b connected to the provider's switch, the switch routes the call using an internal trunk (not pictured). Otherwise, the switch routes the call through the PSTN 105, e.g., directly to a third-party's switch 106 or via the provider's IXC 121 or a third party's IXC 120.
The PSTN 105 is a circuit-switched network, which typically means that it reserves the necessary bandwidth all along the route taken by a call in progress, even when the call does not need all of the reserved bandwidth. Multiplexing techniques, which are well known in the art, are used to send multiple digitized telephone calls simultaneously over the same trunk 108. Switches within the PSTN 105 exchange signaling information used to, e.g., information used to create, route, terminate, and account for telephone calls (among other functions) over a path that is physically or logically distinct from the data paths, and most commonly do so using the protocol known in the art as Signaling System #7 (“SS7”).